Great month lead to an abrupt ending..

It sucks when someone changes their mind about us, but I don't see how much of what you wrote is really pointing to insecure behavior on her part? But rather maybe some insecurity or mismatched expectations on your part. 

"Although she wasn’t really into flirting or talking about deep feelings" - How deep of feelings should a person be sharing on a first date?

"She never has been in a serious relationship and maybe none at all. Her social media was very bare bones"  She is only 24, how many serious relationships should someone be in during that time? Did you express any curiosity behind why she hadn't?  I dislike social media so I view this as a green flag. I also heavily filter out what strangers can see. I will not add someone to social media until 5+ dates. 

"Then 2 hours after the date she responds to a question I sent and then in the same message she sent “I just have to be upfront with you. You’re an amazing person and I’ve enjoyed our time together, but it’s going to work out”  - This is a very mature and secure response. After 3 dates she doesn't owe you any explanation at all. I've had tons of great dates with men where something came out along the way that was a major values mismatch and after sitting on it I ended it. I did not tell them why, it didn't feel necessary.  I sent them a similar type of message and we went on our way.  While it is disappointing that she didn't respond further, again that isn't an insecurity. She stated her piece, you stated yours and then she disengaged. 

I'm really not seeing how she has attachment issues, or anything else going on? The first few dates should be focused on having fun and getting to know someone.